It calculates the drainage directions with the method of the
maximal steepest descent slope, choosing it among 8 possible directions around the selected pixel.
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<br>
<h3>Inputs</h3>
<ul>      
<li>the file containing the matrix of elevations (obtained with pitfiller)</li>
</ul>
<br>
<br>
<h3>Output</h3>
<ul>      
<li>the file containing the matrix of the drainage directions</li> 
</ul>
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<br>
<h3>Notes</h3>
The maximal slope direction is chosen among the 8 possible directions
and codified with numbers ranging from 0 to 8. Such method derives from the one originarily used by D. Tarboton in his Phd thesis. However the outlets are marked with value 10 (beware: many other programs assume it).
